1. Field of the Invention
The present invention relates to a metallic support for an exhaust gas purifying catalyst for an internal combustion engine.
2. Description of the Prior Art
As an example of a conventional metallic support for an exhaust gas purifying catalyst, a metallic support disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 4373/1981 has been known. The metallic support is manufactured as follows: A flat plate and a corrugated plate are superimposed and wound in a roll shape to form a honeycomb body, and then the honeycomb body is accomodated in an outer cylinder made of metal. In the metallic support, the flat plate and the corrugated plate of the honeycomb body as well as the honeycomb body and the outer cylinder are usually joined integrally by brazing.
Further, the metallic support is made into an exhaust gas purifying catalyst by forming a catalyst carrier layer on surfaces of honeycomb passages and loading noble metal into the catalyst carrier layer. The exhaust gas purifying catalyst is disposed in an exhaust system of an internal combustion engine, thereby purifying hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) and the like in an exhaust gas. Since it has been know that it is advantageous to make the areas of the honeycomb passages as large as possible in a limited volume, the thickness of the flat plate and the corrugated plate are made as thin as possible so far as the strengths thereof are maintainable.
Here, the exhaust gas passing through the honeycomb body exhibits a higher flow velocity at a central portion of the honeycomb body than at an outer peripheral portion thereof. Accordingly, in the metallic support, the honeycomb body comes to exhibit a temperature distribution being higher at the central portion thereof and lower at the peripheral portion thereof because of the contact with the high temperature exhaust gas, the heat generated by the catalyst reactions and the heat radiation from the outer cylinder to the environment. The temperature distribution results in a difference between the expansion amounts of the honeycomb body and the outer cylinder and a difference between the contraction amounts thereof. Since the movements of the expansion and the contraction of the honeycomb body in a radial direction and an axial direction are restricted by the outer cylinder, and since the thickness of the flat plate and the corrugated plate constituting the honeycomb body are remarkably thinner than that of the outer cylinder, the honeycomb body is subjected to a thermal stress. As a result, the corrugated plate of the honeycomb body is deformed plastically at an outer most circumferential portion thereof by the repeated expansion and contraction, it comes to suffer from metal fatigue, and eventually it breaks.
In order to avoid the drawback, a metallic support is proposed in Japanese Unexamined Utility Model Publication (KOKAI) No. 194436/1987. In the metallic support, one end of a honeycomb body is joined to an outer cylinder, and the other end thereof is not joined thereto, namely the other end thereof is made into a free end. According to the metallic support, the expansion and the contraction of the honeycomb body in an axial direction are not restricted by the outer cylinder substantially. However, the expansion and the contraction of the honeycomb body in an essential radial direction are restricted by the outer cylinder at the joined end of the honeycomb body as it is restricted in the above-mentioned conventional metallic support. Consequently, there is a fear for breaking a corrugated plate of the honeycomb body at the joined end thereof.
Further, another metallic support is proposed in Japanese Unexamined Utility Model Publication (KOKAI) No. 77634/1988. The metallic support has a honeycomb body whose flat plate has a thickness larger than that of a corrugated plate. In the metallic support, since the rigidity thereof is secured by the flat plate, the amounts of the expansion and the contraction are reduced. Further, since the corrugated plate deforms as a whole in a uniform manner substantially, breakage due to a localized deformation can be prevented from happening. However, since the thickness of the flat plate has been made thicker, the flat plate has exerted an increased resistance to an exhaust gas passing through an exhaust system of an internal combustion engine. Therefore, such an arrangement is not favorable in view of the exhaust performance and the pressure loss.